1. Field of the Invention
This invention relates to marine jet propulsion systems and, more particularly, to improved marine jet propulsion systems designed to operate more efficiently.
2. Description of the Related Art
A typical marine jet propulsion system includes an inlet duct, a pumping means, and a nozzle. The inlet duct delivers water from under the hull to a low volume, high speed pumping means which is coupled to a gasoline powered, internal combustible engine. The pumping means forcibly delivers the water through the nozzle thereby propelling the water craft through the body of water in which the water craft moves.
Heretofore, high revolution, gasoline powered engines have been used in marine jet propulsion systems due to their lower costs, the availability of a wide variety of different horsepowers, their ability to be directly connected to a pumping means and to provide sufficient high RPM required by the pumping means. Due to the relatively high RPM produced by these engines, high speed pumping means are commonly used in such systems. Unfortunately, these high speed pumping means operate most efficiently when a small volume of water under relatively high pressure is delivered thereto. Because only a relatively small amount of water is required by these pumping means, water craft manufacturers, heretofore, have not been concerned with the size or the efficiencies of the inlet duct.
One goal of manufacturers in the marine jet propulsion system industry is to develop jet propulsion systems which are more efficient and provide improved performance and fuel economy. In the prior art, it has been generally accepted that the highest propulsion efficiency for a propulsion system is achieved when a large mass of water is accelerated a very small increment of velocity. In order to achieve high propulsion efficiency with jet propulsion systems, large pumping means and large diameter nozzles must be used. Unfortunately, the manufacturers in the industry have not been able to overcome the increased hydraulic inefficiencies which develop in the large pumping means, large nozzles, and inlet ducts which offset any gains in propulsion efficiency. An improved marine jet propulsion system which uses a large pumping means and a large nozzle to achieve greater propulsion efficiency and which overcomes the hydraulic inefficiencies in the pumping means, nozzle and inlet duct would be very desirable.